The present invention relates to a device for detecting and measuring sound through various media such as walls, car doors and the like, and in particular, to an improved intensity acoustic calibrator.
The use of microphones to detect and measure sound through various media is well known in many arts. Sound detecting and measuring can be used for such varied applications as the detection of flaws in buildings which allow sound to pass between rooms, and the amount of noise leaking through a sound barrier designed to shield residential areas from major highways.
Typically, the sound monitoring device will have two or more microphones which help the user to determine intensity and location of sound. Thus, the user is able to locate sound leaks and take corrective measures if necessary.
In order for the sound monitoring devices to be accurate, the microphones must be periodically calibrated to ensure that they are taking accurate measurements. In the past, this calibration was typically performed in a laboratory. The microphones to be calibrated were placed in a sound cavity, as shown in FIG. 1A and then adjustments were made to calibrate the microphones. This device 10 comprises an elongate tube 14 with an open first end 18 and a pair of microphone holes 22 and 26 at an opposing second end 30. The diameter of the tube is typically about 1.5 inches. In order to conduct a residual intensity test, speaker 32 emits a sound into the tube 14 and a first microphone 34 is tested, followed by a second microphone 38. The positions of the two are then switched and the test repeated. The average of the two tests provides an idea of the phase differential. However, as has been appreciated by those skilled in the art, with this device 10 attenuation of the transverse wave often showed up as phase differential, decreasing the reliability of the test.
Additionally, this method of calibration had other significant drawbacks which inhibit the reliability of the readings obtained by these microphones. The monitoring devices are rarely used in the laboratory. Rather, they are typically used at varied environments and locations. Because temperature, humidity and other environmental factors have a significant impact on the microphones, a pair of microphones which may have been properly calibrated in a laboratory may not be accurately calibrated for a cold, humid environment, such as on a boat, etc. The length of time since the last calibration is also significant: the longer the period of time since the last calibration, the less reliable the results.
In an attempt to resolve these concerns, a device 50 was developed to enable field testing. A simplistic representation of the device 50 is shown in FIG. 1B. The device 50 includes a pair of sound chambers 54 and 58 with an acoustic resistance 62 therebetween. A speaker 66 is placed in one of the chambers, and a microphone 70 and 74 is placed in each chamber. With such a device, the phase differential may be more accurately determined.
Unfortunately, the device generally only works to about 1 kHz, as the frequency is limited by the geometry. New standards adopted by many countries now require testing devices to be calibrated between 63 Hz and 6.3 kHz (ISO 1045). Because the devices currently available are not capable of testing microphones through such a range, it is common to use the electrostatic actuator test. In this test, a high voltage A/C signal (i.e. 800 V) is used to simulate an acoustic signal.
In light of the above, there is a need for an apparatus and method which enables the in situ acoustic calibration of microphones. Such a system will enable calibration under the varying environmental conditions which will be present during actual use of the microphones, and will prevent a significant time lag between the time at which the microphones were calibrated, and the time at which they are used. Such a system will also enable testing through the entire range required by ISO 1043 and related world standards.